1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the production of aromatic hydrocarbons from naphtha.
2. General Background
Aromatic intermediates BTX (benzene, toluene and xylenes) are obtained principally from petroleum naphtha, using a combination of processes to form and recover the desired aromatics. Catalytic reforming generally is the heart of an aromatics complex, producing a mixture of principally aromatics and paraffins to be processed further by some combination of aromatics extraction, dealkylation or disproportionation, adsorption or crystallization, isomerization and fractionation. The various steps were combined to address the issues of achieving high aromatics purity, balancing the product slate in favor of the relatively higher demand for benzene and xylenes, and dealing with the ethylbenzene contained in the mixed xylenes stream. Substantial improvements have been effected in individual processes contained in such aromatics complexes, particularly in catalytic reforming efficiency for aromatics production and in isomerization for conversion of C.sub.8 aromatics.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke. The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins has been widely disclosed in recent years, but commercialization has been slow.
BTX aromatics produced by catalytic reforming generally are subjected to solvent extraction to remove paraffins, naphthenes and other hydrocarbons. In some cases, when the catalytic reforming process is operated at very high severity particularly on lower-cyclic feedstocks in a manner to convert essentially all of the heavier nonaromatics to aromatics or to lighter compounds, C.sub.8 and heavier aromatics may be separated by fractionation without extraction. In any event, aromatics recovered from catalytic reformate by extraction are fractionated to recover pure benzene, toluene and C.sub.8 aromatics.
C.sub.8 aromatics which have been synthesized and recovered in an aromatics complex contain a mixture of the three xylene isomers and ethylbenzene. Para-xylene normally is recovered in high purity from the C.sub.8 aromatics, for example by adsorption or crystallization, and ortho-xylene often is recovered although its markets are more limited. Meta-xylene generally comprises the largest proportion of reformate-derived C.sub.8 aromatics, but rarely is recovered in pure form and often is isomerized to increase the yield of para- and/or ortho-xylene. Separation of ethylbenzene from the xylenes by superfractionation or adsorption is very expensive, and ethylbenzene therefore generally is converted in some manner to other products in a process to isomerize associated xylenes.
Since ethylbenzene is relatively difficult to convert in a xylene-isomerization process, catalysts for the upgrading of C.sub.8 aromatics to improve isomer distribution ordinarily are characterized by the manner of processing ethylbenzene. A concomitant of older isomerization technology was the transalkylation of ethylbenzene with resulting product loss to heavy aromatics. One modern approach to C.sub.8 -aromatics isomerization is to react the ethylbenzene in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function to effect hydrogenation to a naphthene intermediate followed by dehydrogenation to form a xylene mixture. An alternative approach is to convert ethylbenzene via dealkylation to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. The former approach enhances xylene yield by forming xylenes from ethylbenzene, but the latter approach commonly effects higher ethylbenzene conversion and thus lowers the quantity of recycle to the para-xylene recovery unit with a concomitant reduction in processing cost. The latter approach also yields a high-quality benzene product.
Some combinations of catalytic reforming and downstream conversion to provide a product enriched in aromatics are known in the art. U.S. Pat. No. 4,053,388 (Bailey) teaches a combination of catalytic reforming and thermal hydrocracking at 1200.degree.-1380.degree. F. of the reformate to obtain a paraffin stream plus benzene-, toluene, and xylene-rich streams; in the thermal hydrocracking, ethylbenzene is converted at a lower rate than are the xylenes. U.S. Pat. No. 4,157,355 (Addison) discloses catalytic reforming followed by hot flash separation and dealkylation of the separator liquid at 1000.degree.-1500.degree. F. to yield preferably benzene. U.S. Pat. No. 4,181,599 (Miller et al.) discloses reforming and separation of the product to yield a heavy reformate fraction, which is upgraded by conversion with a ZSM-5 catalyst into a BTX-enriched gasoline product.